Concrete driveways, garage floors, sidewalks, steps, and porches can take a beating from frost, freeze-thaw, weather, and traffic. If your concrete is chipped, cracked, looking old and worn, or it has settled and slopes the wrong way, do you have to tear it out? Can you pour concrete over existing concrete and give it a new life?
Old, worn, chipped, or cracked concrete can be resurfaced if it is still structurally sound and the added layer doesn’t create other issues. Clean the old surface, form it up, coat with a bonding agent, pour or place the new concrete, and finish as desired. The new layer should be good for decades.
In this article, we’ll discuss pouring concrete over existing concrete, the minimum thickness a resurfacing should be, and how to make it bond. We’ll also provide a step-by-step guide for pouring concrete over concrete so you’re better prepared to tackle your concrete facelift project.
Can You Pour Concrete Over Existing Concrete?
Ancient concrete has lasted for millennia. Modern concrete came into use in the last 200 years, but when protected and maintained, has demonstrated longevity that is promising. Concrete exposed to the elements and daily wear and tear can become cracked, pitted, chipped, and worn.
You can pour concrete over existing concrete. It is an ideal solution for worn or settled concrete provided it is structurally sound and the added thickness won’t pose a problem with openings and drainage. Structurally sound concrete may have cracks and chips or have settled over time. Adding a layer of concrete on top will bring new life to an old surface.
There are many reasons you may wish to pour concrete over concrete. It saves time, resources, and money, provides a solid base for the pour, and means no removal and disposal concerns. Resurfacing existing concrete can add new life, finish, and even color to worn, chipped and cracked surfaces. Plus, it extends the life of the original concrete. However, if the concrete has multiple through cracks or has heaved or pitched out of alignment, it may not be worth saving and should be removed.
The overall size of the concrete may determine if it’s worth saving or tearing out. It is important to determine if the concrete is structurally sound before making a decision. Non-Destructive Testing (NDT), taking core or cube samples, or a UPV (Ultrasound Pulse Velocity) test are three ways to check the structural integrity but require the use of special equipment and a Structural Engineer.
The rebound hammer test is a common NDT method used to assess the hardness of concrete, which in turn is interpreted as its strength. It can also easily be done on different areas to test for uniform strength across a pad. The test involves a spring-driven hammer or weight and a scale that measures the rebound.
An alternative hammer test is to strike the concrete surface squarely with a hammer face. If it bounces, the compressive strength of the concrete is good. If the hammer doesn’t rebound much or it indents or pulverizes the concrete, it isn’t sound and should be removed. Additionally, a screwdriver dragged over the concrete surface should leave a white streak, if it flakes or chips the surface, there may be scaling or spalling issues that can pose concerns too.
Will New Concrete Stick to Old Concrete?New concrete will not stick to old, dry concrete. In order for new concrete to adhere, it requires a bonding agent. Old concrete that is structurally sound should be cleaned of stains, paint, oil, dust, dirt, and loose or flacking material prior to applying a bonding material.
The old concrete is dry and will draw moisture from the new, altering the water to cement ratio and weakening the pour. Any resulting bond between old and new concrete will be brittle and short-lasting at best. Water needs to permeate into the old concrete prior to covering it with new concrete, so the new bonds into the old surface. Note, there shouldn’t be puddles of water on the old surface.
Applying a thin, wet, scratch coat layer of soupy sand-cement mix or water-cement rubbed onto the surface and into cracks or chips prior to pouring the new layer are other ways to get new concrete to stick to old. Concrete is porous and will absorb some moisture from the scratch coat, making it more likely to form a bond with new concrete.
An alternative to mechanical scratch coats is commercially available chemical or epoxy bonding agents. They are commonly ‘painted’, rubbed, or sprayed onto the old surface and into cracks and pits. The bonding medium becomes tacky after several minutes, and the new concrete is poured or placed on top.
Pro Note: Adding an overlay of 3-inches or less requires the use of bonding solutions or agents to get the two layers to form one. Layers thicker than 3-inches require a bond breaker to separate the two surfaces.
Minimum Concrete Thickness Over Existing Concrete
The minimum concrete thickness over existing concrete depends on location, use, and available clearances. Resurfacing a driveway or garage floor commonly requires greater thickness than a porch or an interior floor may. Heavy traffic surfaces may also need to be thicker to allow for reinforcing mesh or steel. If there isn’t the elevation available, removal may be the better option than going with a thinner overlay.
A rule of thumb is the thickness of the new layer should be 2 and 1/2 times the size of the aggregate or ballast used. A 3/4″ aggregate requires a minimum pour thickness of about 2-inches, while 1/4″ stone only needs a 5/8” pour. A smaller stone chip or fiber aggregate may allow for an even thinner layering. Additionally, a sand-cement or cement only overlay may be as thin as 1/4″.
While most ready-mix concrete over concrete pours are a minimum of 1-1/2” to 2-inches, the area to be resurfaced may also influence the pour thickness. Re-sloping a settled porch may require a 2-inch thickness at one edge and only 1/4” to 1/2″ at another. Select an aggregate that works for the thinnest and use it for the whole pour.
How to Pour Concrete Over Concrete
Once you’ve determined that the old concrete is structurally sound and there is enough clearance to pour concrete over concrete, it’s time to take action. Check the weather forecast to ensure there is a dry window of opportunity and collect the materials you’ll require. It’s always a good idea to have plastic sheeting or tarps to cover the pour in case the forecast changes. The size of the pour will influence the choice of ready-mix vs hand-mixed.
Here are 10 easy steps for pouring concrete over concrete:
Step 1: Clear around the perimeter for forms and stakes.
Step 2: Clean off stains, dirt, paint drops, oil, loose material, and dust.
Step 3: Install and stake forms. Forms should be treated to prevent concrete from sticking to them.
Step 4: Cut rebar or mesh to fit if using it.
Step 5: Mix-up the scratch coat and apply, or apply bonding medium, or wet the old concrete until it is saturated.
Step 6: Place rebar or mesh if using it.
Step 7: Pour or place ready-mix, or mix-up concrete and pour or place.
Step 8: Screed or float the concrete level and let it set-up.
Step 9: Edge, groove, and finish the surface as desired.
Step 10: Cover if necessary and allow to cure for 48 hours or more before removing forms.
Pouring Concrete Next to Existing Concrete
You may choose to enlarge an existing pad, or cut out weakened concrete from structurally sound, and pour new material. Pouring concrete so it joins the old and forms a strong bond can be done in different ways. However, the thickness of the existing concrete affects how the new pour is joined with the old, so they lift, move, or settle together.
One method is to drill holes in the old concrete face for rebar “dowels” or pins to be set into. This practice works best for concrete 6” thick or greater, as it can compromise and weaken the old concrete if it is 4” thick or less. The dowelled face of the old concrete is treated with a bonding material to facilitate the melding of the old and new concrete.
Concrete 4” or less thick should be shelved. Shelving is a process of providing support for the old and new edge by creating a shelf or beam for support. Excavate 6” to 12” horizontally under the old concrete and 4” to 6” or 12” down to form a lip.
Do the same along the edge of the new pour too and create a beam cavity – it is common to use rebar to reinforce the ‘beam’. The face of the old material is coated with a bonding agent, and the new slab poured.
What if My Old Concrete Has a Lot of Cracks in It?
Cracks in concrete may indicate that the structural integrity of the mass is compromised, and it should be removed. If the material isn’t compromised, resurfacing will cover the cracks and prevent moisture causing more hydraulic or freeze-thaw damage.
Small, shallow surface cracks, crazing, or scaling are fine to cover with another layer of concrete once loose material is removed. Wide or through cracks or cracks resulting in uneven or unlevel sections identify structural issues that a new layer of concrete probably won’t survive.
Cracks in concrete are common and occur due to shrinkage, design flaws, lack of reinforcement, and subsoil movement. The type of cracking often helps determine if the pad is compromised or sound. The most common causes are environmental exposure, weathering, and the freeze-thaw process. The type of crack usually identifies the cause and is often indicated by the pattern and nature of the cracks.
There are two types of shrinkage cracks; drying or curing cracks, and plastic shrinkage cracks. Both types can be avoided, minimized, or controlled with proper preparation and procedures during the layout and curing stages.
Drying Shrinkage or Curing Cracks
Cracks that occur after the concrete has set and hardened but before it is fully cured are drying cracks. The shrinkage is due to water evaporating out mass, causing it to contract.
Contraction seams are often put before the concrete fully sets, or cut into the slab within 24-hours of the pour, to control the location of the cracking. Without contraction cuts or joints, you may see transverse, map, or pattern cracking where you don’t want them as the pad cures.
Plastic Shrinkage Cracks
Concrete that dries too quickly is subject to short cracks, crazing, crackling, scaling, or spalling issues. If moisture bleeds and evaporates too quickly as the concrete hardens and cures, the change in volume can cause cracks and other problems. Slowing the evaporation process using evaporation retarders like damp blankets, sand, wind screens, plastic sheeting, or sprays of water.
Thermal cracks are caused by heat expanding the mass and causing it to push against other surfaces or structures. The force can cause the concrete to crack. The use of rebar and expansion joints with a compressible material such as rubber, lumber, or asphalt help prevent thermal cracks by resisting or absorbing the compression forces.
Freeze-thaw is another form of thermal force that results in heaving cracks. As the ground freezes, it can lift with enough force to crack or break concrete that isn’t able to move as the ground does. Rebar and expansion cracks also help minimize these types of cracks.
Flexural cracks – flexing or settling cracks – commonly run vertically through the concrete and along tension zones. The cracks usually are wider at the surface and narrow as they go through the concrete. They are more common in concrete without adequate reinforcement, or where subsurface erosion or weakness occurs – often along backfilled zones or where disturbed soil meets undisturbed ground. Although they may start at an edge, they frequent the mid-span of the slab.
Other Types of Cracks
Heaving cracks may be caused by the freeze-thaw process. They can be caused by tree roots and the lifting or settling of subsurface utilities such as sewer pipes or culverts too.
Cracks may also occur if the rebar begins to corrode due to moisture or chemical reactions in the concrete expanding from within the mass. Another type of cracking is caused by overloading – something heavy like a concrete truck rolling over a curb or concrete pad can exceed the strength of the concrete, and crack or crush it.
Does Concrete Overlay Last?
The durability of concrete overlays depends upon the thickness of the new layer, the condition of the under-laying concrete, exposure to the elements, use, and how well it bonded in the first place. Structurally sound concrete is necessary, otherwise, the cracks in the old will make their way through the new.
If the bond isn’t good, footfall impact or vehicle traffic and the elements will quickly damage it. The thickness of the overlay determines how quickly the other factors may affect the finish.
A well-bonded overlay spread over clean, structurally sound concrete will easily last 10 to 15 years. There are also numerous examples of them lasting 2 to 3 decades or more. Whether a thin or thick layer, protected or exposed to harsh climatic conditions and regular vehicle or foot traffic, an overlay is a great option.
Overlays commonly range from 1/4″ to 2” in thickness, but there are instances of 6” to 8” overlays in some repurposing situations. A 1/4″ overlay can last as long as a 2” or 6” one provided it is installed properly over structurally sound and clean concrete. Care and maintenance are often the key to longevity in concrete, the same as it is in cars, trucks, and humans.
Pouring new concrete over old worn concrete is a great way to give it a clean, fresh look that should last for decades. Ensure the old is structurally sound and clean of dirt, dust, and loose material.
Use an appropriate bonding agent, pour and finish the new, and allow it to cure properly. A much better option to tearing out the old and starting over. Hopefully, your old concrete is structurally sound so you can take advantage of what you now know about pouring concrete over concrete.